Electron microscope

ABSTRACT

Electron optical aberrations of an energy filtering system of an energy filtering transmission electron microscope (EFTEM) are automatically corrected under computer control to set up the EFTEM for use. Optics of the electron microscope preceding an energy filter are used to scan the beam at the entrance to the filter in a pattern corresponding to a defined geometry. The beam can either be finely focused to yield a spot at each position visited during the pattern scan, or the beam can be spread out and imprinted with a well-defined intensity distribution, such as normally occurs due to passage of the beam through a specimen, so that its relative scanned displacements can be assessed using cross-correlation techniques. In the case of the finely focused beam, electron images of the scanned pattern directly yield a spot pattern image. Deviation of the recorded spot pattern image from the defined scan geometry reflect the imaging aberrations introduced by the energy filter. In the case of the spread out beam, post-filter electron images of the scanned beam are cross-correlated with an image of the beam taken without scanning yielding cross-correlation peak images that give the effective displacement of each scanned beam position due to the aberrations/distortions of the filter. Summing the cross-correlation peak images again yields a spot pattern image that is equivalent to that obtained in the focused beam case. Deviations of the recorded spot pattern image from the defined scan geometry are analyzed to assess and subsequently correct aberrations introduced by the energy filter.

The invention relates to an electron microscope provided with anelectron source for the generation of a beam of electrons; anenergy-dispersive element for the dispersion of the paths of electronshaving a different kinetic energy; an accelerating tube for theacceleration to a predetermined beam energy of the electron beam fromthe electron source to a specimen to be studied with the aid of anelectron microscope; a plate mounted between the energy-dispersiveelement and the specimen, in which a selection slit is provided at rightangles to the dispersive direction of the dispersive element for theselection of dispersed electrons having a kinetic energy within adesired energy interval; and source imaging electron optics forobtaining an image of a source in the plane of the plate comprising theselection slit.

Such an electron microscope is known from practice. Generally speaking,with an electron microscope it is possible to see smaller structuresthan is possible with light, making use of the wave characteristics ofthe electrons to produce an image of the specimen, and of the extremelyshort wavelength of the electrons, allowing the observance of the smallstructures. An electron microscope further makes it possible to analyzethe specimen by determining the transmission of electrons through thespecimen. It is also possible to analyze the specimen by measuring theenergy loss of the electrons after they have passed through thespecimen. In general this energy loss is an extremely small fraction ofthe kinetic energy of the electrons in the beam. When using this methodof determining the energy loss, it is possible to scan the specimen witha sharply defined electron beam by means of a point by point analysis ofthe specimen. The smaller the diameter of the beam on the specimen, thebetter the resolution of the analysis. The above summary is notexhaustive but merely gives an impression of the possibilities and thegreat importance of an electron microscope. For the variouspossibilities diverse types of electron microscopes, such as thetransmission electron microscope (TEM) and the scanning electronmicroscope (SEM), have been developed.

The demands on the analytical properties and the spacial resolution ofthe various electron microscopes are continuously increasing. Apossibility of improving them is to introduce an energy-dispersiveelement in the beam's path from the electron source to the specimen.This makes it possible to reduce the electron's energy dispersion in thebeam, which dispersion is mainly caused by the different velocities withwhich the electrons leave the electron source. The reduction of thisenergy dispersion makes it possible to not only increase the energyresolution of energy loss spectra to be determined, but also to improvethe spacial resolution of the microscope, since the setting parametersof the electron optics used will only be optimal for a specific kineticenergy of the electrons. To operate the electron microscope properly,the setting parameters have to be aligned optimally.

One difficulty when using the energy-dispersive element is that faultyimaging of the electron source may occur. For example, an astigmaticimage may be formed, which means that there are different focusingplanes for the dispersive and the non-dispersive direction of thedispersive element. The image of the electron source has to lie in theplane of the plate with the selection slit which is ultimately used forthe selection of the electron beam's appropriate energy interval. Such aplate may, for example, consist of two plate halves between which theslit is formed. In order to render the image stigmatic, it is possibleto add a so-called stigmator to the source imaging electron optics. Sucha stigmator may be integrated with the energy-dispersive element. Tomake the image stigmatic and to focus it in the plane of the platecomprising the selection slit, the setting parameters of the stigmatorand a number of other elements of the source imaging electron opticsalso have to be optimized.

The specific embodiments of the various elements applied will not beentered into. Such elements are known in the electron microscopy andthey use setting voltages and currents as setting parameters.

The optimization of these setting parameters poses a problem with theknown electron microscope. When aligning this microscope, the electronsource is imaged as well as possible in the plane of the selection slit,and a magnified image is subsequently projected onto the fluorescencescreen of the electron microscope. However, it is not possible to assesswhether the source is imaged properly in the plane of the selectionslit, as this image is observed via a further image on the fluorescencescreen. Another disadvantage is that the electron microscope'smagnifying optics for imaging the first image on the fluorescence screenhave to be aligned separately, which is time consuming and cannot becarried out automatically.

It is the object of the invention to provide an electron microscope ofwhich the setting parameters in respect to various optical elements forthe electron beam can be aligned fully and efficiently. To this end theinvention provides an electron microscope, characterized in that inaddition to the selection slit, the plate comprises a plurality offurther apertures useful for the determination of the cross-sectionalform of the beam; and in that the electron microscope comprises meansfor the determination of the intensity of the beam being transmittedthrough and/or onto the plate so that subject thereto, settingparameters of the energy-dispersive element and the source imagingelectron optics can be aligned.

The apertures in the plate allow the beam to pass in a particularcrosswise direction with respect to the direction of the beam, with theshape of the apertures having to be such that the beam's intensity canbe determined in different cross directions. Said transmitted beamintensity can be measured by the means mentioned, so that said intensitycoupled to a particular direction is known. In this manner immediateinformation is obtained regarding the dimensions and the position of theimage in the plane comprising the selection slit. This is extremelyimportant for the precise alignment of the setting parameters forobtaining an optimal image of the electron source in the plane of theselection slit.

The invention further provides the advantage that the optimization ofthe setting parameter can be performed automatically by interposing acalculation and control unit with an algorithm controlling the settingparameters in accordance with the particular beam intensities. Anautomatic optimization process further contributes to obtaining anoptimal and quick alignment of the various setting parameters.

Preferably the membrane is positioned directly after theenergy-dispersive element and the source imaging electron optics. Thismanner of positioning produces an image on the membrane that is notinfluenced by elements that do not need to be optimized, which resultsin the most precise alignment.

In a preferred embodiment the electron microscope is characterized inthat the dimensions of the apertures are in the nanometric range; and inthat the plate constitutes a thin membrane placed at a position wherethe electron's kinetic energy is so low that it can be blocked by thethin membrane. Such small structures ensure that the obtainedinformation relating to the location of the image in the plane of theselection slit has a high resolution, which further contributes to anextremely precise optimization. With such small structures it isnecessary that the membrane they are made into is thin, having athickness in the order of the dimensions of the apertures. Too thick aplate results in long and narrow channels through the plate. Suchchannels cause electrons to be scattered on their walls, which must beavoided as much as possible to prevent imaging problems.

It should be noted that from a contribution by the inventors at the EMAG'97 conference (Electron Microscopy Analysis Group Conference,Cambridge, UK, 1997) placing a membrane having a narrow slit directlyafter an energy-dispersive element is already known. However, theconsiderations that culminated in the present invention of applyingapertures with dimensions in the nanometric range and placing themdirectly after the elements to be optimized, are specific for theoptimization of the alignment of their setting parameters.

To facilitate the alignment, a favourable embodiment is characterized inthat the plate is permanently positioned, and in that the source imagingelectron optics comprise at least one deflection means allowing theelectron paths to be deflected in both the dispersive direction of thedispersive element and a direction perpendicular thereto. Such anembodiment is especially suitable for automatization.

The different apertures may have various shapes. In a preferredembodiment the further apertures comprise at least one additional slitat an angle with the selection slit, so that the intensity of the beamcan also be determined in the dispersive direction, for instance, tomake it possible for the image in the non-dispersive direction to alsobe projected in the plane of the selection slit.

In order to be able to determine the form of the beam in the dispersivedirection only, the additional apertures comprise at least one slitparallel to the dispersive direction of the dispersive element.

For the composition of different configurations of slits the additionalapertures comprise at least one slit parallel to the selection slit. Anumber of slits may be arranged in a star shape with the beam duringoptimization being able to move about the centre point of said starconfiguration, and in a pattern comprising at least one rectangle.

The other apertures further comprise preferably at least one openingmuch smaller than the cross-sectional dimension of the electron beam sothat an exact image of the beam can be obtained, and so that only asmall part of the image of the electron source is available whenmeasuring specimens, while the edge effects of the source are blocked.

At the same time, the other apertures may comprise at least one openingwhich is much larger than the cross-sectional dimension of the electronbeam in order to allow the whole beam to pass, or in order to obtain aslowly integrating signal when the beam is being moved from the plateinto the opening.

In a preferred embodiment similar apertures are provided havingdifferent dimensions, so that openings can be selected whose dimensions,for example the width of the slit, match a desired energy interval orwhose dimensions correspond with the size of the image of the electronsource, to obtain a transmitted beam of maximum brightness.

In possible embodiments of the electron microscope according to theinvention, the means for the determination of beam intensity transmittedthrough and/or onto the plate comprising the apertures include a currentdetector which, viewed in the direction of the beam, is placed after theplate and connected thereto is a current meter for measuring theelectrons passing through the apertures; and/or a current detectorwhich, viewed in the direction of the beam, is placed before the plateand connected thereto is a current meter for measuring the electronsreflected by the plate; and/or a current detector connected with theplate for measuring the incidence of electrons onto the plate, but notreflected by the plate.

The invention will be explained in more detail with reference to theappended drawings in which identical parts are indicated by the samereference numbers, and in which

FIG. 1 shows an electron microscope according to the prior art, whereinimaging of the electron source is schematically indicated onsuccessively the selection slit and the fluorescence screen;

FIG. 2 shows an electron microscope according to the invention, whereinimaging of the electron source on a membrane comprising the apertures isschematically indicated;

FIG. 3 shows an electron microscopic image of a membrane comprising theapertures of nanometric dimensions, as used in a preferred embodiment ofthe electron microscope according to the invention; and

FIGS. 4a to 4 d show different measured intensity profiles, as shown onthe right-hand side of the Figures, during scanning of the electron beamover the different apertures of the membrane shown in FIG. 3, as shownon the left-hand side of the Figures.

The electron microscopes shown in the FIGS. 1 and 2 comprise an electronsource 10. This may be formed by a Schottky field emission source,having an extremely small effective source capacity which is typicallyapproximately 50 nm (nanometre). The electrons leaving the sourcepossess a certain range in the kinetic energy, which range is for theSchottky source typically 0.4-0.9 eV (electron-volt).

The spacial resolution of the electron microscope is limited among otherthings by chromatic aberration, that is to say that the imagingcharacteristics of the microscope depend on the specific energy of theelectrons in the electron beam. When imaging, all the energies withinthe energy dispersion of the electron beam will form images in differentplaces which, if high spacial resolution is desired, must be prevented.Further, energy dispersion in the electron beam is also disadvantageouswhen making an analysis wherein the energy loss is determined byelectrons that have passed through a specimen (not shown). The energyresolution that can be attained in such an analysis will also depend onthe energy dispersion in the beam.

In order to limit this dispersion of the electrons' kinetic energy inthe beam, an energy-dispersive element 20 is provided between theelectron source 10 and the specimen. Since the energy-dispersive element20 is the most effective when the kinetic energy of the electrons islow, this element 20 is located in the vicinity of the electron source10, because the electrons are eventually accelerated to a voltage ofapproximately 1 keV (kiloelectron-volt) or considerably higher (forexample 400 keV). To this end the specimen is placed on an electricpotential with a relatively low absolute value, and the electron sourceon a high negative electric potential, and the electrons are acceleratedin an interposed acceleration tube 30. The energy-dispersive element 20will then also be positioned at this high negative electric potential,in order not to or only slightly to accelerate the electrons between theelectron source 10 and the dispersive element 20, and to keep thekinetic energy of the electrons in the dispersive element 20 low. Forthe final selection of a desired energy interval, a plate 40 comprisinga selection slit has been placed between the energy-dispersive element20 and the specimen, to allow the transmission of electrons having adesired kinetic energy. In general the plate 40 is placed at the end ofthe acceleration tube 30 at an electric potential with a relatively lowabsolute value, so that it can be manipulated, for example, tofacilitate the alignment of the width of the selection slit of a plate40 consisting of two different halves between which the selection slitis formed.

Further, electron optics 50 have been provided near the electron source10 for imaging the electron source 10 in the plane of the plate 40comprising the selection slit. In the ideal case, due to theenergy-dispersive element 20, the different paths of electrons havingdifferent kinetic energies will form different images, but in the planeof the plate 40. At the same time, however, the use of theenergy-dispersive element 20 causes astigmatic focusing, which meansthat the images in the dispersive and the non-dispersive direction ofthe energy-dispersive element 20 are formed in different, successiveplanes. This results in a limitation of the spacial resolution of theelectron microscope. The astigmatic focusing can be cancelled by meansof adding a so-called stigmator to the electron optics of themicroscope. Such a stigmator is not separately shown, but in theembodiments according to the FIGS. 1 and 2 it is incorporated in theelectron optics 50. The electron optics 50 comprising the stigmator canalso be embodied as a whole with the energy-dispersive element 20,wherein an energy-dispersive element 20 embodied as Wien-filter may beprovided with different electric and magnetic multipoles, so as tosimultaneously serve as focusing element.

The above-mentioned optical elements of the electron optics 50 and theenergy-dispersive element 20 are known, and as such are not part of thepresent invention. They will not be discussed further. Important is,however, that these optical elements must be aligned so that an optimalsetting can be obtained for them and consequently for the electronmicroscope. To this end said optical element's setting parameters formedby setting voltages and currents require optimal alignment.

The prior art electron microscope according to FIG. 1 achieves this withthe aid of magnifying optics 85, whereby a magnified image of theelectron source 10 on the plate 40 comprising the selection slit, isprojected onto the fluorescence screen 75. Thus the image on the plate40 is not observed directly, but indirectly. This makes it extremelydifficult to determine whether the electron source 10 is imaged exactlyin the plane of the plate 40. Likewise, it is hardly possible to achievean optimal setting for the electron microscope based solely on viewingthe image on the fluorescence screen 75, and it is a very painstakingprocess.

The electron microscope according to the invention shown in FIG. 2,solves this problem by means of a plate 40 comprising the selection slitand a plurality of additional apertures providing information withregard to the cross-sectional shape of the electron beam. In thepreferred embodiment shown, the plate 40 is a membrane 40′ comprisingapertures with dimensions in the nanometric range (nano-structures) forthe provision of information with a high spacial resolution. Further,the membrane 40′ is positioned directly after the optical elements whosesetting parameters have to be aligned when optimizing. It is nowpossible to optimize said parameters independently of the rest of themicroscope.

Using known lithographical techniques, the nano-structures can be etchedinto a 100 nm-thick membrane of silicon nitride (Si₃N₄). This membranelayer is placed on a silicon wafer into which an opening of 0.1×0.1 mm(millimetre) was etched to completely lay bare the membrane in thissection. Both sides of the silicon wafer are thinly coated with platinumto aid the conduction of electrons from the electron beam falling ontothe silicon wafer and the membrane. After the various operations, thesilicon wafer is broken into pieces of 2.5×2.5 mm, and such a piece issubsequently mounted into the electron microscope. FIG. 3 shows thevarious structures that are etched in a preferred embodiment of themembrane 40′. Later on these structures will be further elucidated inconnection with FIG. 4.

The intensity of the beam transmitted through the nano-structures andonto the membrane can be measured with the aid of the current detectors60 and current meters 61 shown in FIG. 2. Involved in this case are,among other things, a current detector 60 and current meter 61 forelectrons transmitted through the nano structures. In the embodimentshown the intensity is not spacial-resolution measured. This does becomea possibility by using a two-dimensional detector. In addition, acurrent meter 61 is shown for measuring the incidence of electrons onthe membrane 40′ that are not reflected, and a current detector 60 andcurrent meter 61 for measuring the incidence of electrons on themembrane 40′, that are reflected.

With the aid of a computer the currents measured with the differentcurrent meters 61 can be read out simultaneously and thus be correlated.Scanning the beam over the nano-structures can also take placecomputer-controlled. By using a suitable computer program-implementedalgorithm and standard control means it is possible to automate theentire optimization process. Such control means and a computer are notshown in the Figures, but the person skilled in the art will be familiarwith these.

In order for the beam to scan over the nano-structures, the electronbeam has to be deflected into a direction approximately perpendicular tothe direction of the beam. This can be achieved through the integrationof deflectors in the electron optics 50. Such deflectors are known assuch. It is possible to use a dispersive element 20 acting as deflector,to scan the beam in its dispersive direction.

The nano-structures shown in FIG. 3 are large openings for the unimpededtransmission of the electron beams when, for example, the dispersiveelement 20 is switched off, or when switching-on the source, the same isnot yet properly focused and only the large openings transmit enoughelectrons for the beam to be detectable. The large rectangular or squareopenings may further be used for so-called “sharp-edged” measurements,wherein the beam is scanned over the edge of the opening. When scanningthe beam from the membrane into the opening, the integral beam intensitygradually increases. After differentiation of such a measurement, thisresults in the beam intensity in the scanning direction of the beam.Such a “sharp-edged” measurement is schematically illustrated in FIG.4a, showing on the left-hand side in perspective the beam on themembrane using a large opening, and indicating the scanning direction.On the right-hand side in FIG. 4a the intensity gradient I is given as afunction of the scanning direction x.

It goes without saying, that there are also selection slits arrangedperpendicularly to the dispersive direction of the energy-dispersiveelement 20 for the selection of a desired energy interval of the beam tobe transmitted. With the aid of said slits, it is possible to determinethe transmitted beam intensity perpendicularly to the dispersivedirection. There are also slits parallel to the dispersive direction forthe determination of the beam intensity in the dispersive direction. Thescanning of a beam over the a slit in the direction x is schematicallyin perspective illustrated on the left-hand side in FIG. 4b; on theright-hand side in FIG. 4d the respective measured intensity gradient Ican be seen.

The different lengths of the apertures can be calibrated simply with theaid of the structures shown, which are arranged in a pattern consistingof various rectangles in which the slits stand at right angle to oneanother. These right-angular structures allow the direct measurement ofthe image in a particular direction.

When the beam is moved about the centre of the star-shaped structure,said star-shaped structures allow the size and quality of the image tobe measured in different directions. By means of the eight-point starshown here, a cross section through the beam can be obtained in three45°-degree directions. Of course, star shapes comprising more directionsmay also be applied. Scanning of the beam over the eight-point starshape and the respective intensity gradient I as a function of thescanning angle φ, are shown in FIG. 4c on the left- and right-hand side,respectively.

In addition there are openings present of a size much smaller than thedimension of the cross section of the beam for obtaining a precise imageof the beam. Such a measurement with respective intensity gradient isshown in FIG. 4d. The intensity is here shown in a shade of grey as afunction of two scanning directions x and y at right angles to oneanother. This kind of measurement is, however, relatively timeconsuming.

It is also possible to use a small opening to obtain a “clean” filteredbeam. Frequently an electron source exhibits weak emission at the edgesof the emission area; this may spoil sensitive measurements. A smallopening allows such emission edges to be removed also in non-dispersivedirections.

Some of the various apertures have different dimensions. For obtainingoptimal brightness, it is thus possible both to select a dimension tomatch the dimension of the electron source and an energy interval thatdepends on the width in the dispersive direction.

The embodiments described above are not to be considered to limit theinvention. Within the scope of the present invention and the appendedclaims the electron microscope may be realized in a variety ofembodiments which all fall within the invention's scope of protection.

What is claimed is:
 1. In an electron microscope provided with anelectron source for generation of a beam of electrons; anenergy-dispersive element for dispersion of paths of electrons having adifferent kinetic energy; an accelerating tube for acceleration to apredetermined beam energy of an electron beam from the electron sourceto a specimen to be studied with aid of an electron microscope; a platemounted between the energy-dispersive element and the specimen, in whicha selection slit is provided at right angles to the dispersive directionof dispersive element for the selection of dispersed electrons having akinetic energy within a desired energy interval; and source imagingelectron optics for obtaining an image of a source in a plane of theplate comprising the selection slit, an improvement wherein in additionto the selection slit, the plate comprises a plurality of furtherapertures useful for determination of cross-sectional form of the beam;and wherein the electron microscope comprises means for determination ofintensity of the beam being transmitted through and/or onto the plate sothat subject thereto, setting parameters of the energy-dispersiveelement and the source imaging electron optics can be aligned.
 2. Amicroscope according to claim 1 additionally comprising a calculationand control unit controlling the setting parameters in accordance withparticular beam intensities.
 3. A microscope according to claim 1wherein the plate is positioned directly after the energy-dispersiveelement and the source imaging electron optics.
 4. A microscopeaccording to claim 1, wherein dimensions of the apertures are in ananometric range and wherein the plate comprises a thin membrane placedat a position where the electron's kinetic energy is so low that it canbe blocked by the thin membrane.
 5. A microscope according to claim 1,wherein the plate is permanently positioned, and in that the sourceimaging electron optics comprise at least one deflection means allowingthe electron paths to be deflected in both the dispersive direction ofthe dispersive element and a direction perpendicular thereto.
 6. Amicroscope according to claim 1, wherein the further apertures compriseat least one additional slit at an angle with the selection slit.
 7. Amicroscope according to claim 1, wherein the additional aperturescomprise at least one slit parallel to the dispersive direction of thedispersive element.
 8. A microscope according to claim 1, wherein theadditional apertures comprise at least one slit parallel to theselection slit.
 9. A microscope according to claim 1, wherein a numberof slits are arranged in a star shape.
 10. A microscope according toclaim 1, wherein a number of slits are arranged in a pattern comprisingat least one rectangle.
 11. A microscope according to claim 1, whereinthe other apertures further comprise at least one opening much smallerthan the cross-sectional dimension of the electron beam.
 12. Amicroscope according to claim 1, wherein the further apertures compriseat least one opening much larger than the cross-sectional dimension ofthe electron beam.
 13. A microscope according to claim 1, whereinsimilar apertures are provided having different dimensions.
 14. Amicroscope according to claim 1, wherein the means for the determinationof beam intensity transmitted through and/or onto the plate comprisingthe apertures include a current detector which, viewed in the directionof the beam, is placed after the plate and connected thereto is acurrent meter for measuring the electrons passing through the apertures.15. A microscope according to claim 1, wherein the means for thedetermination of beam intensity transmitted through and/or onto theplate comprising the apertures include a current detector which, viewedin the direction of the beam, is placed before the plate and connectedthereto is a current meter for measuring the electrons reflected by theplate.
 16. A microscope according to claim 1, wherein the means for thedetermination of beam intensity transmitted through and/or onto theplate comprising the apertures include a current meter connected withthe plate for measuring the incidence of electrons onto the plate, butnot reflected by the plate.